Neisseria meningitidis (meningococcus) is a Gram-negative spherical bacterium. Current meningococcal vaccines are also based on capsular saccharides. These include monovalent serogroup C conjugate vaccines and 4-valent conjugate mixtures for serogroups A, C, W135 and Y. There is currently no useful vaccine authorised for general use against serogroup B (‘MenB’).
One antigen which has been proposed for use in immunising against MenB is the factor H binding protein (fHBP). This antigen has also been called protein ‘741’ (SEQ IDs 2535 & 2536 in ref. 34), ‘NMB1870’, ‘GNA1870’ [refs. 1-3], ‘P2086’, ‘LP2086’ or ‘ORF2086’ [4-6]. The protein has been well studied. It is naturally a lipoprotein and is expressed across all meningococcal serogroups. The structure of fHbp's C-terminal immunodominant domain (‘fHbpC’) has been determined by NMR [7]. This part of the protein forms an eight-stranded β-barrel, whose strands are connected by loops of variable lengths. The barrel is preceded by a short α-helix and by a flexible N-terminal tail.
The fHBP antigen falls into three distinct variants [8] and it has been found that serum raised against a given family is bactericidal within the same family, but is not active against strains which express one of the other two families i.e. there is intra-family cross-protection, but not inter-family cross-protection. Thus reference 8 proposes to combine different variants of fHBP into a single vaccine composition, thereby increasing strain coverage, either as a mixture of separate proteins or as a fusion protein of the different variants (the latter being ‘tandem proteins’).
Reference 9 also reports a fHBP tandem protein (pages 18-19 of reference 9). This tandem protein was purified and mixed with aluminium phosphate as an adjuvant, but it is reported not to adsorb well to the adjuvant. Good adsorption of the antigens is desirable, and it has been found that such mixed fHBP proteins readily adsorb if aluminium hydroxide is used as an adjuvant instead.
A problem when using aluminium hydroxide as an adjuvant, however, is that it can degrade certain antigens. For instance, reference 10 reports that it can hydrolyse H. influenzae type B conjugate vaccines, even at low temperatures, thus leading to reduced efficacy. Similarly, hydrolysis of S. typhi Vi capsular saccharide in the presence of aluminium hydroxide is reported in reference 11. Thus it can be desirable to formulate antigens using an adjuvant based on aluminium phosphate, particularly if the adjuvanted vaccine may be mixed (either during manufacture or at the time of use) with an antigen that may be susceptible to damage by an aluminium hydroxide e.g. a conjugated bacterial capsular saccharide.
Thus there is a need to provide formulations of fHBP, and in particular of multiple fHBP variants, in which the fHBP(s) is/are adsorbed to an adjuvant but which do not require aluminium hydroxide.